Low-temperature hydrothermal alteration of intra-caldera tuffs, Miocene Tejeda caldera, Gran Canaria, Canary Islands

Abstract

The Miocene Tejeda caldera on Gran Canaria erupted similar to 20 rhyolite-trachyte ignimbrites (Mogin Group 14-13.3 Ma), followed by similar to 20 phonolitic lava flows and ignimbrites (Fataga Group 13-8.5 Ma). Upper-Mogan tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogin ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid-rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar, plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid-rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The delta O-18 values of the altered intra-caldera tuffs are significantly higher than in unaltered extracaldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extracaldera ignimbrites have delta D values between -110%. and -173%., which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The delta D values of the altered intra-caldera tuffs range from -52%. to -131%., with ambient meteoric water at the alteration site estimated at ca. -15%.. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock delta D values of around -45%, given that Delta Dclay-water is ca. -30%. at 100 degrees C, and decreases in magnitude at higher temperatures. All altered tuff samples have delta D values that are substantially lower than -45%, indicating interaction with a meteoric water source with a delta D value more negative than -15%., which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200-250 degrees C. (C) 2008 Elsevier B.V. All rights reserved.